The present invention relates to a manufacturing method for spent fuel storage members that accommodate and store spent nuclear fuel assemblies whose burnup has been finished, and form a cask or a rack. This invention also relates to mixed powder that is suitable for manufacturing such spent fuel storage members.
A nuclear fuel assembly at the final stage of a nuclear fuel cycle, at which burnup of the fuel has been finished so that the fuel can no longer be used, is referred to as spent nuclear fuel. Currently, such spent nuclear fuel is stored and controlled in a storage facility until it is subjected to chemical reprocessing. A storage system using a fuel pool, for example, is executed by sinking an SUS rack with rectangular pipes tied in a bundle into the pool and accommodating spent fuel assemblies in these rectangular pipes Accordingly, this system satisfies requirements such as a cooling effect, a shielding effect, and subcriticality.
In recent years, a start has been made using a member, which is obtained by adding boron to a stainless material, as rectangle pipes forming the rack. By using such rectangular pipes, neutron absorbing materials disposed between the rectangular pipes can be omitted, therefore, the gaps between the rectangular pipes can be narrowed. Accordingly, the number of rectangular pipes insertable into the pit of the pool increases, which makes it possible to increase the number of spent fuel assemblies to be accommodated in the pit.
Such rectangular pipes are applicable to various types of storage system using a cask, a horizontal silo, a pool, a bold, or the like. However, if the rack is formed with these pipes, a large number of rectangular pipes are required to be manufactured. Therefore, technology for enabling efficient productivity of the rectangular pipes has been demanded. Further, each rectangular pipe needs to absorb neutrons produced from the spent fuel assembly, therefore, the solidity of its structure is required.
Further, a plate type of rack is known other than the rack with the rectangular pipes used for storage of the spent fuel assemblies. This plate type of rack also requires efficient productivity and the solidity of its structure.
It is an object to provide a manufacturing method for such rectangular pipes.
According to one aspect of this invention, powder of aluminum is mixed with powder of a neutron absorbing material, and this mixed powder is premolded by means of cold isostatic pressing (CIP). The premolded body is canned and subjected to sintering. Accordingly, a billet as a preprocessed product for molding a spent fuel storage member is finished. By premolding the mixed powder, nonuniformity in molding density can be reduced. In order to take out the billet from the can, the can is subjected to outer cutting and end face cutting. Further, it is preferable to perform sintering by means of hot pressing or hot isostatic pressing (HIP). The method for Dummy HIP or atmospheric sintering may also be used other than these processes. The spent fuel storage member includes rectangular pipes that form a basket, or plate members that form a plate type rack. The sintering is performed by means of hot pressing or HIP, which makes it possible to manufacture a spent fuel storage member with higher quality.
According to another aspect of this invention, the canning step may be omitted and vacuum sintering may be performed. If the canning step is omitted, no machining such as outer cutting is required after vacuum sintering. Therefore, manufacturing the billet may be facilitated. Further, vacuum hot pressing is the most adequate for the vacuum sintering. In addition to this process, vacuum dummy HIP may be used. By performing the sintering through the vacuum hot pressing, a low-cost, yet high-quality spent fuel storage member can be manufactured.
According to still another aspect of this invention, a premolded body is subjected to electric discharge sintering, which makes it possible to sinter it for a shorter period as compared to ordinary sintering. Accordingly, the spent fuel storage member can efficiently be manufactured. Since the step of canning is omitted, the need for machining such as outer cutting is eliminated, which makes it possible to manufacture the spent fuel storage member at lower cost. Further, it is preferable to use electric discharge plasma sintering for electric discharge sintering. That is because sufficient sintering can be performed by removing a passive film from aluminum by the energy due to this electric discharge plasma sintering. In addition to this electric discharge plasma sintering, thermal plasma sintering can also be used.
Further, by extruding the billet manufactured by the method, a rectangular pipe or a rod as the spent fuel storage member can easily be manufactured.
Further, it is clear that a neutron absorbing material such as boron or a boron compound agglomerates and segregates during sintering if its average particle diameter is as small as about some micrometers. Therefore, boron whose average particle diameter is ten-odd micrometers or more is used in the ordinary sintering. However, if the average particle diameter is large, the strength of the spent fuel storage member is decreased.
Therefore, in the manufacturing method according to the invention, the aluminum powder is mixed with the powder of the neutron absorbing material by mechanical alloying. Various types of ball milling can be used for this mechanical alloying. Each particle of the aluminum powder is changed to a flat shape during a milling process by ball milling. The particles of the neutron absorbing material are milled by ball milling to become considerably small as compared to the initial average particle diameter, and are dispersed into the aluminum matrix while being rubbed thereinto. Accordingly, the neutron absorbing material can be dispersed finely and uniformly, thus improving the mechanical strength of the spent fuel storage member.
Further, by performing ball milling, the balls are worn out while colliding against each other, which causes the component of the balls to be mixed into the powder. In order to solve this problem, each ball formed with an element as its main component, which is supposed to be added originally, is used, and this element will be added to the powder through attrition of the balls. By doing this, it is possible to omit some steps of the manufacturing process.
Further, the powder is mixed using a powder mixing device that generates high-velocity airflow. Accordingly, the powder particles of the neutron absorbing material in the high-velocity airflow are made finer by collision, and are sunk into and attached to the surfaces of the aluminum powder particles that are also in the high-velocity airflow. Accordingly, the powder particles of the neutron absorbing material can be dispersed finely and uniformly into the aluminum powder particles, thus improving the mechanical strength of the spent fuel storage member. Further, the rotating speed of a rotary container used for the powder mixing device that generates the high-velocity airflow is preferably set in a range from 70 to 80 m/sec. That is because, if the number of revolutions is low, the neutron absorbing material is not attached to aluminum and the remaining absorbing material changes the mixing ratio. On the other hand, if the number of revolutions is high, aluminum is welded inside the device due to heat produced at the time of collision.
According to still another aspect of this invention, by sintering the neutron absorbing material powder, the particles of the neutron absorbing material powder during sintering can be prevented from their agglomeration. The particles of boron are milled to finer ones by mechanical alloying and are dispersed into the particles of aluminum powder so as to be rubbed into the aluminum powder particles. Thus, the spent fuel storage member manufactured by using such powder becomes excellent in its mechanical strength.
According to still another aspect of this invention, by sintering the neutron absorbing material powder, the particles of the neutron absorbing material powder during sintering can be prevented from their agglomeration. The particles of boron are milled to finer ones by a powder mixing device that generates high-velocity airflow, and are sunk into and attached to the surfaces of the aluminum powder particles. Thus, the spent fuel storage member manufactured by using such powder becomes excellent in its mechanical strength.